Lithium-ion batteries (LIBs) have attracted extensive attention in the past two decades for a wide range of applications in portable electronic devices such as cellular phones and laptop computers. Due to rapid market development of electric vehicles (EV) and grid energy storage, high-performance, low-cost LIBs are currently offering one of the most promising options for large-scale energy storage devices.
Currently, electrodes are prepared by dispersing fine powders of an active battery electrode material, a conductive agent, and a binder material in an appropriate solvent. The dispersion can be coated onto a current collector such as a copper or aluminum metal foil, and then dried at an elevated temperature to remove the solvent. Sheets of the cathode and anode are subsequently stacked or rolled with the separator separating the cathode and anode to form a battery.
The lithium-ion battery manufacturing process is sensitive to moisture. A battery with high water content leads to serious attenuation of electrochemical performance and affects stability of battery. Therefore, environmental humidity must be controlled strictly for the production process of LIBs. Most of the LIBs are produced in an environment with less than 1 percent humidity. However, significant cost is incurred because of the stringent moisture-free process. To address the moisture sensitive issue of electrode assembly, it is important to dry the electrode assembly prior to electrolyte filing so as to reduce the water content in the battery.
Chinese Patent No. 104142045 B describes a method of drying an electrode assembly of LIBs. The method comprises heating an electrode assembly under vacuum at a temperature of 30-100° C.; filling the oven with dry air or inert gas; repeating these two steps for 1-10 times. This method provides the electrode assembly with a water content between 430.5 ppm and 488.1 ppm.
Chinese Patent Application No. 105115250 A describes a method of drying an electrode assembly of LIBs. The method comprises heating an electrode assembly under vacuum at a temperature of 85±5° C.; filling the oven with hot, dry nitrogen gas; repeating these two steps for 10-20 times. This method provides the electrode assembly with a water content of less than 200 ppm.
Chinese Patent No. 102735023 B describes a method of drying an electrode assembly of LIBs. The method comprises heating an electrode assembly under vacuum at a temperature of 20-70° C.; filling the oven with dry air or nitrogen gas; repeating these two steps for 5-50 times. This method provides the electrode assembly with a water content between 110.1 ppm and 137.2 ppm.
Chinese Patent No. 103344097 B describes a method of drying an electrode assembly of LIBs. The method comprises heating an electrode assembly under vacuum at a temperature of 75-85° C.; filling the oven with a non-oxidizing gas; heating the electrode assembly to 75-85° C.; vacuum drying the electrode assembly again. However, this method does not provide the water content of the dried electrode assembly for evaluating the drying process.
None of the above patent references discloses any binder composition in the electrodes for evaluating the relationship between the drying profile and binder composition. In addition, the water contents of the electrode assemblies as dried by the existing methods range from a hundred ppm to several hundreds ppm, which may affect the cycling stability and rate capability of LIBs.
In view of the above, there is always a need to develop a method for drying electrode assemblies of LIBs to low water content. Also there is always a need for electrode assemblies having a low water content and good electrochemical performance.